It isn't exactly obvious how the phone behaves when BT SAP connected. Current phones can handle multiple simultaneous BT connections but that may not be enough.
I had to try it with my Nokia E90, should be pretty similar to the N95 but I'm not sure N73 would have the same S60 version (should be mentioned at
www.nokia.com).
Early BT SAP implementations killed the phone user interface pretty completely. A S40 phone would just show it is BT SAP connected and all you can do is select "quit SAP". The current S60 allows access to most applications, except some that are cellular connectivity dependent. An example being email. Odd to me that you cannot even open the application to read previously received emails. The again, the browser application opens but only to use WLAN, it is not able to use GPRS from the MB BT device (I tried with my laptop and it looks like it does not even support GPRS dial-up, unlike the N616 BT SAP car kit would do).
I've actually tried the internal E90 navigation application before when BT SAP had been active. The internal one works fine.
Next I disabled the internal E90 GPS and connected the phone to a BT GPS device. It did not appear to be straightforward to get the internal navigation application connect with the external BT GPS even if "Position" application clearly showed the phone was receiving GPS data from the external BT GPS device. Perhaps I did not activate it right but once the phone navi found the GPS data from the external device, I went to the car, activated BT SAP connection to the car and the phone navi was still working fine with the external GPS.
It looks like the GPS part works fine either way (internal or external GPS). I forgot to try navigation announcements. The point being that audio should in this case come from the phone speakers, the BT SAP connection cannot carry BT audio to let you hear the navigation voice guidance from the car speakers. Also I remember earlier using this BT GPS in another phone where the navi audio was totally lost when the phone was connected to a BT HF kit (a bit different from this SAP though). If possible, I would test the Tom Tom application in the car before buying but it could actually work quite fine.
